Vehicles have been developed to perform an engine stop when specific engine idle-stop conditions are met and then to automatically restart the engine when restart conditions are met. Such idle-stop systems enable fuel savings, reduced exhaust emissions, reduced vehicle noise, and the like. In some idle-stop systems, engine speed is controlled during an engine restart by loading the engine via an alternator mechanically coupled to the engine. However, during engine restarting a substantial amount of current is required by a starter from a battery to start the engine. Consequently, when a higher amount of current is drawn from the battery, the battery voltage may be lowered and the mechanical load provided by the alternator to the engine may change in an unexpected and/or unpredictable manner.
One example of an engine starting system is shown by Kusafuka et al. in U.S. Pat. No. 7,471,069. Herein, an alternator, a starter, and a voltage raising device (e.g., a DC/DC converter) are connected to the positive electrode of a system battery such that during an engine restart, power from the battery is used by the starter to start the engine. At the same time, power from the DC/DC converter is used to operate audio and navigation systems. The DC/DC converter helps to buffer the audio and navigation systems from reduced battery voltage during engine starting by providing a regulated voltage output that is less sensitive to changes in battery voltage.
However, the inventors herein have recognized a potential issue with such an approach. As one example, in the given configuration of the electrical components described in U.S. Pat. No. 7,471,069, as the age of the battery increases, and/or a condition of the battery degrades, the voltage supplied by the battery to the alternator field coil excitation circuit during engine cranking and run-up is decreased (e.g., due to voltage droop during engine starting). The alternator field coil excitation circuit can vary the amount of battery voltage applied to the alternator field coil up to the battery voltage. As a result, the maximum voltage applied to the alternator field coil may be changed from one cranking event to another cranking event so that the alternator field current is inconsistent. A decreasing alternator field current can also decrease the amount of mechanical load that the alternator applies to the engine during starting. Consequently, the load the engine is subject to from the alternator can vary from start to start as battery voltage varies. As a result, engine speed may flare and overshoot a desired engine speed. Such an engine speed may be noticeable and objectionable to a driver. In addition, when the average voltage applied to the alternator field is reduced, the alternator is capable of outputting less current to the battery and ancillary electrical loads. Therefore, the response time of a power assist steering system or other electrical devices electrically coupled to the alternator may be degraded.
As one example, the above issue may be at least partly addressed by an engine starting system comprising an engine, an engine starter, a first battery in electrical communication with the engine starter during an engine start, and an alternator mechanically coupled to the engine. The alternator may have an alternator field coil excitation circuit that is electrically buffered from the first battery during an engine start, the alternator field coil excitation circuit in electrical communication with a power source other than the first battery during the engine start.
In one example, a vehicle engine starter circuit may include a battery configured to power a starter during an engine start. The battery may also be electrically coupled to a DC/DC converter (or DC/DC converter based device). The DC/DC converter may be configured to provide a regulated voltage output which may be used to power one or more electrical components and auxiliary loads during the engine start (e.g., vehicle lights, radio, etc.). A field coil excitation circuit of an alternator may also be coupled to the output of the DC/DC converter such that the alternator field coil excitation circuit is added as an additional load to the DC/DC converter. In one example, the field coil excitation circuit may be a linear voltage regulator. In another example, the field coil excitation circuit may be a pulse width modulation circuit that controls an average voltage that is applied to the alternator field coil. During an engine start, a switch arranged in parallel across the DC/DC converter may be opened so that the alternator field coil excitation circuit is electrically buffered from the battery via the DC/DC converter from start to start. Thus, during the engine start, even if the battery has aged, a substantially consistent average voltage may still be applied to the alternator field coil via the alternator field coil excitation circuit since the input to the alternator field coil excitation circuit is maintained at a substantially constant voltage level by the DC/DC converter. Consequently, a mechanical load applied to the engine by the alternator can be made more predictable and consistent from start to start, thereby enabling improved control of engine run-up speed.
In an alternate example, the DC/DC converter may be removed and a second alternate power source (e.g. a battery) configured with a directional current flow limiting device, which limits current flow from the second alternative power source to the first battery, may be electrically coupled to the alternator field coil excitation circuit. In this way, the alternator field coil excitation circuit can be coupled to the alternate power source to a power to the alternator field such that the alternator field coil and field coil excitation circuit are electrically buffered from the main system battery. Consequently, the alternator field coil and alternator field coil excitation circuit may be buffered from the effects of voltage droop due to battery aging and/or a degraded battery condition. By providing a more predictable and consistent alternator mechanical load to the engine during engine starting, the quality of engine restarts may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.